Neil Sperry Says Chill Tulips 45 Days Before a Late-December Planting: The Calendar and Bulb-Count Math

In his October 22 "Gardening This Weekend" column, Texas horticulturist Neil Sperry put a deadline on a job most warm-climate gardeners put off. Buy tulips and Dutch hyacinths now, he wrote, and refrigerate them at 45°F for at least 45 days. Then plant them in the second half of December. Without that artificial winter, he warned, they won't bloom normally in Texas.

The reason is simple. Tulips and hyacinths need weeks of steady cold to form a proper flower stalk. Where winters stay mild, the ground never gets cold for long enough, so the refrigerator has to fill in. Sperry's advice comes down to two numbers, 45 days and a late-December planting window, and the real question is what they mean for this weekend. Here is the math.

Count Backward From the Planting Window

Start with the date you plan to plant and subtract the chilling time. Sperry's window runs from December 16 to December 31:

• Plant December 16 − 45 days = start chilling by November 1.
• Plant December 31 − 45 days = start chilling by November 16.

In other words, bulbs that go into the refrigerator this weekend can be planted on any day in Sperry's window. Bulbs that go in after mid-November will either miss the window or get less than the minimum. If you are still shopping, that two-week cushion is the useful number. Put the purchase date and the planting date on the calendar now, before the bulbs disappear into a crisper drawer.

Why "At Least" Matters

Sperry gives 45 days as a minimum, and other sources ask for much more. Bulb supplier Longfield Gardens recommends 10 to 14 weeks (70 to 98 days) of chilling for tulips in Zones 8 to 10. It says that shorter chilling can produce short stems or flowers that don't fully open. The two pieces of advice don't really conflict. Forty-five days is roughly where normal blooming starts, and more chilling generally gives taller, better-formed flowers.

Here is the same backward math for the longer chilling periods, starting this weekend:

• Chill from November 1 for 45 days: ready December 16.
• Chill from November 1 for 10 weeks: ready January 10.
• Chill from November 1 for 14 weeks: ready February 7.

If you want the fullest chill, plan to plant in January instead of December. Longfield's own guidance puts warm-climate planting in late December or early January. Don't stretch storage indefinitely, though. Longfield suggests getting refrigerated bulbs into the ground within about four months.

Protect the Refrigerator Weeks

Ruining 45 days of chilling in the last week is easy. These three rules keep the cold you have already banked:

• Keep fruit out. Ripening apples, pears, and stone fruit give off ethylene gas. Longfield warns that ethylene can make the flower bud inside a bulb abort, so you get leaves and no bloom. Use a spare beverage fridge if you have one. If not, empty the produce drawer of fruit for the duration.
• Let them breathe. Store bulbs in mesh onion bags or loosely filled paper bags, not sealed plastic, which traps moisture and invites mold. Check for dampness every couple of weeks.
• Check the temperature. Sperry calls for 45°F. Longfield says a typical refrigerator at 35 to 40°F also works. Put a cheap thermometer on the shelf for a day to find out what yours actually holds. The bulbs should be cold but never frozen.

How Many Bulbs Does the Bed Take?

The fastest way to overspend on bulbs, or to run out of refrigerator space, is to buy without counting. On a square grid, bulbs per square foot = (12 ÷ spacing in inches)².

• 6-inch spacing: (12 ÷ 6)² = 4 bulbs per square foot.
• 5-inch spacing: (12 ÷ 5)² = about 5.8 bulbs per square foot.
• 4-inch spacing: (12 ÷ 4)² = 9 bulbs per square foot.

A 3-by-6-foot strip along a walkway is 18 square feet. That takes 72 bulbs at 6 inches apart or 162 at 4 inches. That difference more than doubles both the bill and the shelf space. Close spacing gives the packed, solid-color look of a display bed, and wider spacing gives a looser one. Choose the look first, then let the math set the order. (Our plant spacing formula guide explains the grid math in more detail, including staggered rows.)

Before the bulbs go into the refrigerator, measure how much room they really take: count how many fill a one-quart container, then scale up to your order. A fridge shelf that seems roomy fills up fast at 160 bulbs.

Planting Depth on the Day

When the chilling is done, go straight from refrigerator to soil. Don't let the bulbs sit warm on the porch for a week. Longfield's rule of thumb is to plant about three times the bulb's height deep, which works out to roughly 6 inches for a typical tulip, measured from the base. If you are already shredding leaves this fall (see our fall leaves mulch math), a light layer over the planted bed helps keep the soil cool and moist. Sperry suggests a 2-inch layer of shredded leaves for perennial beds after frost.

Quick Recap

• Sperry's rule for Texas: at least 45 days at 45°F, then plant tulips and Dutch hyacinths in the second half of December.
• Count backward: start chilling by November 1 to plant December 16, or by November 16 to plant December 31.
• Forty-five days is a minimum. A supplier recommends 10 to 14 weeks for tulips in Zones 8 to 10, which moves planting into January or early February.
• Keep ripening fruit out of the bulb refrigerator, use breathable bags, and check the real temperature.
• Bulbs per square foot = (12 ÷ spacing)²: 4 at 6 inches, 9 at 4 inches. Count before you buy.

With chilled bulbs, the calendar decides the outcome more than the soil does. Put the start date on the refrigerator door this weekend, and the spring display in late winter is mostly settled.